Solar energy can be used and stored by the efficient production of long-lived photo-induced charge separation--a state achieved in photosynthetic systems by the formation of a long-lived radical pair. A number of artificial systems have been reported that efficiently undergo photochemical charge transfer, unfortunately, the thermal back electron transfer often proceeds at an appreciable rate, limiting the utility of these systems. What is needed is a system which has very efficient photoinduced charge transfer, and forms a charge-separated state which is long lived in air. The charge separation in these systems typically involves a redox reaction between a photo excited donor and a suitable acceptor, resulting in the production of radical ion pairs illustrated by the formula: EQU D+hv.fwdarw.D* (1a) EQU D*+A.fwdarw.A.sup.- +D.sup.+ (1b) EQU D.sup.+ +A.sup.- .fwdarw.D+A (2)
The cation and anion generated in this way are better oxidants and reductants, respectively, than either of the neutral ground-state molecules. To harvest the light put into this system, the oxidizing and reducing power of the photo-generated species must be used before the electrons are transferred back (equation 2) generating the starting materials. It is desirable to control this photochemically unproductive thermal fast back electron transfer reaction. One method has been to incorporate the donors and acceptors into solid matrices.
Design and characterization of chemically sensitive interfaces and thin films has focused on attempts to mimic the highly efficient processes observed in biological systems, many of which occur in or at membranes. Thus, a key goal in this area is the fabrication of an artificial system for the conversion of solar energy into chemical or electrical energy. This approach to energy conversion can take a number of forms, ranging from the design of novel photovoltaic devices to the search for an efficient and cost-effective method to photochemically convert liquid water to gaseous hydrogen and oxygen. Properly designed systems can use the photoinduced charge separation to generate a photocurrent.
In a process to generate chemical energy, D.sup.+ and A.sup.- are used to drive uphill chemical reactions, such as the oxidation and reduction of water, respectively. In order to generate electrical energy the same species can be used as the anode and cathode of a photocell. In order for either of these processes to be efficient back electron transfer (equation 2) must be prevented. In order to retard back electron transfer it is important to control both the structural and electronic properties of the system. In natural reaction centers this goal is achieved by fixed geometrical arrangements of electron donors, intermediate carriers and electron acceptors within the membrane. In artificial systems , the electron donors and acceptors with chosen redox potentials may be arranged in a fixed geometry using simple self-assembly techniques.
The individual components in the charge separated state have the appropriate potentials to carry out the reduction and oxidation of water. Unfortunately, these direct reactions are kinetically limited, such that catalysts are required to overcome the kinetic barriers. Colloidal platinum particles are ideal catalysts for the reduction of water to give H.sub.2. In systems used for photoreduction of water, the close contact of high potential radicals formed in the compounds and Pt particles is advantageous, because electron transfer from reduced viologen to Pt particles should compete effectively with back electron transfer. These platinum particles may be present in the reaction solution, incorporated into the structure of the compositions, or both.
Compounds which can carry out reduction reactions, using hydrogen gas as their reducing equivalents, are useful as catalysts for the conversion of mixtures of hydrogen and oxygen to hydrogen peroxide. Hydrogen peroxide is a very large volume chemical. The United States annual production is greater than 500 million lbs. Several processes have been patented for the production of hydrogen peroxide, which depend on the two following reactions. The goal is to promote reaction (3) and retard reaction (4): EQU H.sub.2 +O.sub.2 .fwdarw.H.sub.2 O.sub.2 (3) EQU H.sub.2 O.sub.2 +H.sub.2 .fwdarw.2H.sub.2 O (4)
A number of catalysts for this conversion have been reported including both homogeneous and heterogeneous catalysts.
Compositions of the present invention are capable of producing a sustained photoinduced charge separation state which renders the compositions useful in solar energy conversion and storage. Multilayer thin films of the present invention composed of donor and acceptor layers produce photocurrents when irradiated with light. In addition, the compositions permit reduction of various metal ions to produce the zero-valence metal in colloidal form entrapped in the matrices of the compositions. These latter matrices containing the zero-valence metal have a variety of uses such as in the decomposition of water to yield hydrogen gas and the sensing of oxygen. In addition, the zero-valence metal matrices can be used in catalysis, as for example in the production of hydrogen peroxide and the oligomerization of methane to form higher hydrocarbons.